U.S. patent number 4,216,045 [Application Number 05/906,340] was granted by the patent office on 1980-08-05 for process for preparation of electrode for alkaline battery.
This patent grant is currently assigned to Sanyo Electric Co., Ltd.. Invention is credited to Yuji Morioka.
United States Patent |
4,216,045 |
Morioka |
August 5, 1980 |
Process for preparation of electrode for alkaline battery
Abstract
A novel process is provided for producing an improved electrode
for an alkaline battery comprising the steps of adding an
unsintered fluorocarbon resin powder to an active material powder
and intimately mixing them together, applying a shear force to said
mixture to thereby form agglomerates capable of retaining said
active material powder in a network structure resulting from a
fiber-formation of said fluorocarbon resin powder, adding an
aqueous solution of a paste to said agglomerates, kneading them to
mutually bind said agglomerates and thus forming a rubbery mass
converting said rubbery mass into a sheet and attaching the sheet
thus formed onto an electrically conductive core. The process
provides an electrode having excellent reactivity and results in
improved production efficiency.
Inventors: |
Morioka; Yuji (Sumoto,
JP) |
Assignee: |
Sanyo Electric Co., Ltd.
(Moriguchi, JP)
|
Family
ID: |
13073470 |
Appl.
No.: |
05/906,340 |
Filed: |
May 16, 1978 |
Foreign Application Priority Data
|
|
|
|
|
May 18, 1977 [JP] |
|
|
52-58064 |
|
Current U.S.
Class: |
156/242;
252/182.1; 428/422; 429/217; 521/54; 525/187; 204/291; 264/104;
264/294; 429/212; 429/222; 525/57 |
Current CPC
Class: |
H01M
4/0435 (20130101); H01M 4/30 (20130101); H01M
4/04 (20130101); H01M 4/0416 (20130101); H01M
4/0404 (20130101); Y02E 60/10 (20130101); Y10T
428/31544 (20150401) |
Current International
Class: |
H01M
4/04 (20060101); H01M 4/30 (20060101); C04B
035/00 (); B29C 019/00 () |
Field of
Search: |
;260/29.6F,37M,900
;428/422 ;204/291 ;264/104,294,127 ;156/62.2,242,306
;429/212,222,217 ;252/182.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Powell; William A.
Assistant Examiner: Gallagher; J. J.
Attorney, Agent or Firm: Armstrong, Nikaido, Marmelstein
& Kubovcik
Claims
What is claimed is:
1. A process for preparation of an electrode for alkaline battery
comprising the steps of:
(a) forming an intimate admixture consisting essentially of an
unsintered fluorocarbon resin powder and a battery electrode
forming active material powder;
(b) applying a shear force to said mixture sufficient to cause the
fluorocarbon resin to be formed into fibers and to form
agglomerates capable of retaining said active material powder
inside a network structure resulting from a fiberformation of said
fluorocarbon resin powder;
(c) adding an aqueous solution of natural or synthetic polymer
paste to said agglomerates;
(d) kneading the resulting mixture to mutually bind said
agglomerates and thus form a rubbery mass;
(e) converting said rubbery mass into a sheet; and
(f) attaching the sheet so formed onto an electrically conductive
core body.
2. The process for preparation of an electrode for an alkaline
battery as defined in claim 1 wherein said fluorocarbon resin is
polytetrafluoroethylene.
3. The process for preparation of an electrode for an alkaline
battery as defined in claim 2 wherein the amount of said
polytetrafluoroethylene is from 0.5 to 2.0% based on the weight of
said active material powder.
4. The process for prepation of an electrode for an alkaline
battery as defined in claim 1 wherein said natural or synthetic
polymer paste is a member selected from the group consisting of
polyvinyl alcohol, polyethylene oxide, methyl cellulose,
hydroxypropyl cellulose and carboxymethyl cellulose and the amount
thereof is from 0.2 to 0.4% based on the weight of said active
material powder.
5. The process for preparation of an electrode for an alkaline
battery as defined in claim 1 wherein said aqueous paste solution
has a viscosity of 50 to 3,000 cps.
6. The process for preparation of an electrode for an alkaline
battery as defined in claim 1 wherein said active material is
cadmium oxide and an alkali metal condensation oxyacid salt is
added to said aqueous paste solution.
7. The process for preparation of an electrode for an alkaline
battery as defined in claim 6 wherein said condensation oxyacid
salt is an alkali metal oxyacid salt of phosphorus, silicon or
arsenic.
8. The process for preparation of an electrode for an alkaline
battery as defined in claim 6 wherein the amount added of said
condensation oxyacid salt is from 0.2 to 2.0% based on the weight
of the cadmium oxide powder.
9. The process for preparation of an electrode for an alkaline
battery as defined in claim 1 wherein said active material is
CdO.
10. The process for preparation of an electrode for an alkaline
battery as defined in claim 6 or 7 wherein said aqueous paste
solution has a viscosity of 50 to 3000 cps.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for producing an electrode of
an alkaline battery and is specifically directed to a process for
producing an electrode having high energy density, good mechanical
strength and excellent reactivity of the active material by the use
of an unsintered fluorocarbon resin as a binder.
In an enclosed type Ni-Cd battery, for example, it is an essential
commercial requirement that the cadmium electrode has a
charge-discharge life of at least 1,000 cycles, an energy density
of at least about 500 mAH/cc and extremely good reactivity with an
oxygen gas for the purpose of gas absorption inside the
battery.
As an electrode satisfying these requirements, a sintered type
electrode is generally known. The electrode consists of a porous
substrate obtained by sintering a nickel powder and then
impregnating the substrate with an active material.
In the sintered type electrode of the above-described type,
however, the nickel powder forming the porous substrate is
expensive. In addition, the porous substrate does not with
certainty participate in the reaction from the standpoint of
battery capacity so that the energy density obtainable is at the
maximum about 600 mAH/cc. Moreover, it is a time-consuming and
troublesome operation to form the porous substrate and to have the
active material impregnated and supported inside pores of the
substrate. For these reasons, the resulting electrode becomes
necessarily expensive.
Apart from the abovementioned sintered type electrode, a paste type
electrode also is known in the art.
The production process of this paste type electrode generally
comprises kneading an active material powder into a paste form
together with an aqueous solution of a paste, dissolving therein a
natural or synthetic polymer paste material such as polyvinyl
alcohol, methyl cellulose, carboxymethyl cellulose, polyethylene
oxide and the like, coating the resulting paste onto an
electrically conductive core body and drying the core to form the
electrode.
In comparison with the sintered type electrode, the paste type
electrode is more advantageous in that the number of production
steps is less and since the major part of the electrode is formed
by the active material which participates in the battery reaction,
the obtainable energy density is by far higher, that is to say, up
to about 700 mAH/cc. However, the paste type electrode is not free
from drawbacks. Since the polymer paste used as the binder is
swellable in water which is one of principal components of an
electrolyte, the electrode does not have sufficient mechanical
strength. Also, the polymer paste is oxidized inside the battery to
form a carbonic acid ion in such an amount as to exert adverse
influence on the battery reaction. Furthermore, since the binding
mechanism of the polymer paste relies on a film-forming action, the
active material is partially covered by the film, thereby lowering
its reactivity.
As a method of improving these drawbacks, U.S. Pat. No. 3,630,781
discloses a novel production method of an electrode similar to the
paste type electrode.
This method is characterized by the use of an unsintered
fluorocarbon resin, e.g., tetrafluoroethylene resin (hereinafter
referred to as PTFE), hexafluoropropylene resin,
chlorotrifluorroethylen resin, vinylidene fluoride resin, and
copolymer variations thereof, as a binder. More specifically, the
method comprises dispersing and homogeneously mixing an active
material powder in a water-soluble dispersion of the PTFE resin
obtained by emulsion polymerization, irreversibly breaking said
dispersion under conditions such that the PTFE resin is not
sintered, applying a shear force so as to cause fiber-formation in
the PTFE resin and thus form a rubbery mass having malleability in
such a state where the active material and the residual water are
retained inside the resulting network structure of the PTFE resin,
forming a sheet from the rubbery mass by an ordinary calender
method and attaching the sheet to an electrically conductive core
to thereby obtain the contemplated electrode.
It is known that when a shear force is applied, the chain-like
molecules of the abovementioned unsintered PTFE resin obtained by
emulsion polymerization cause fiber-formation and entangle with one
another to form a network structure and thus exhibit a certain kind
of binding effect.
The PTFE resin has extremely good oxidation resistance and chemical
resistance and does not form a carbonic acid ion which occurs when
the aforementioned polymer paste is used. Since the binding action
of the PTFE resin does not rely on the film-forming action as in
the case of the polymer paste, the active material is maintained in
good contact with the electrolyte as well as with the gas. In order
to obtain the same mechanical strength for a given electrode, the
weight required for the resin is about 1/2 and the volume is about
1/4 in comparison with the polymer paste (the density of the PTFE
resin being about two times the polymer paste).
As mentioned above, the method of U.S. Pat. No. 3,630,781 is
definitely advantageous when compared with the conventional paste
type electrode.
In the method of U.S. Pat. No. 3,630,781, however, the active
material powder is mixed with the aqueous dispersion of the PTFE
resin and the dispersion is then irreversibly broken. Accordingly,
in the sense of "mixing", the homogeneous state is established
between the active material and the fluorocarbon resin, but from
the viewpoint of the battery performance and mechanical strength,
the excessive homogenity is not only meaningless but also involves
a possible problem that a phenomenon similar to the film-forming
action of the polymer paste can occur due to the water-repellency
of the fluorocarbon resin.
In addition, the dispersion contains residual surfactant used for
the purpose of keeping the water-repellent PTFE resin in the
emulsified and dispersed state in water and residual catalyst used
for the emulsion polymerization. In order to eliminate the adverse
influence of these agents on battery performance, an additional
step of removing these additives is necessary after formation of
the electrode.
During the abovementioned production steps, it is necessary at the
step of forming the rubbery mass to partially reduce the water
content of the dispersion and set the residual water content to a
predetermined level in order to apply the shear force. Since the
removal of the water is carried out by such means as evaporation by
heating, control is not easy. During the step of applying the shear
force after removal of water, the energy loss of the shear force is
great due to lubricative effect of the water, and it is difficult
to form the network structure of the PTFE resin having a
predetermined strength with a high level of accuracy.
Because of the viscosity of the fibers of the PTFE resin, the
surface of the electrode thus finished is adherent. Hence, the
active material is sometimes peeled off upon contact with other
components, such as the separator during the assembly of the
battery. For this reason, the adherent electrode is extremely
difficult to handle from the aspect of assembly. It is of further
significance that when an active material having high reactivity
with water such as cadmium oxide is used as the starting material,
cadmium oxide readily reacts with the water during the mixing step
with the aqueous dispersion and such reaction affects adversely the
battery performance.
In other words, as illustrated by the following reaction
scheme;
cadmium oxide readily reacts with the water whereby cadmium oxide
having a large density (8.15 g/cm.sup.3) is converted to cadmium
hydroxide (density=4.79 g/cm.sup.3). This increase in the volume
takes place during the production of the electrode and
consequently, the packed density per unit volume of the active
material is reduce, thereby resulting in a decrease in the energy
density. For this reason, the upper limit of the energy density is
at the maximum about 750 mAH/cc in accordance with this method.
On the other hand, in addition to the aforementioned aqueous
dispersion of the PTFE resin, an unsintered PTFE resin of such a
type is also commercially available. In such a resin, detrimental
components adversely affecting the battery performance such as the
surfactant, the catalyst and the like are perfectly removed by
aggregating the resin after the emulsion polymerization.
U.S. Pat. No. 3,898,099 proposes a production method of an
electrode using an unsintered PTFE resin powder of the
abovedescribed type.
The method of U.S. Pat. No. 3,898,099 is different from that of the
abovementioned U.S. Pat. No. 3,630,781 in that it uses a PTFE resin
powder and non-aqueous lubricant in place of the aqueous dispersion
of the PTFE resin.
Since this method uses non-aqueous lubricant, it is possible to
prevent cadmium oxide from being converted to cadmium hydroxide.
Another advantage of this method is that the PTFE powder does not
contain the additives affecting adversely the battery performance
such as the surfactant, the catalyst, etc., thus the method does
not require the additional step of removing these additives.
According to the method of U.S. Pat. No. 3,898,099, however, the
non-aqueous lubricant is added in an excessive amount in order to
obtain a homogeneous mixture of the active material and the PTFE
resin powder. In removing the lubricant, therefore, it is difficult
to control the residual liquid amount to a predetermined level. It
is further necessary in this method to perfectly remove the
lubricant after the formation of the electrode and for that
purpose, drying is carried out. However, this drying treatment must
be carried out at a temperature below the sintering temperature of
the PTFE resin. Hence, the lubricant to be used is necessarily
restricted to volatile lubricants such as mineral spirits, for
example. This inevitably results in harm of organic solvents and
inevitably requires use of a large-sized production apparatus.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to
provide a process for producing an improved electrode for an
alkaline battery which uses an unsintered fluorocarbon resin powder
as a binder.
According to the process of the present invention, it is possible
to obtain an electrode having excellent reactivity and to improve
production efficiency.
The present invention provides a process for producing an electrode
for an alkaline battery which comprises adding an unsintered
fluorocarbon resin powder to a battery electrode forming active
material powder and mixing them together, applying a shear force to
this powder mixture to cause fiber formation within the
fluorocarbon resin powder and to form agglomerates capable of
retaining the active material powder inside the network structure
of the fluorocarbon resin, adding a predetermined amount of an
aqueous paste solution to the agglomerates, kneading the resulting
mixture to form a rubbery mass having high malleability, forming a
sheet from the rubbery mass, and attaching the resulting sheet onto
an electrically conductive core body and drying the same.
The present invention further provides a process for producing a
cadmium electrode for an alkaline battery wherein when the active
material is cadmium oxide, and a condensation oxyacid salt is added
to the aqueous paste solution.
In the present invention, the scope of the fluorocarbon resin is
the same as described in U.S. Pat. No. 3,630,781.
The abovementioned PTFE resin powder is commercially available in
the form of secondary particles of a particle size of about 400
microns consisting of primary particles of about 0.3 microns
agglomerated with one another. On the other hand, the particle size
of the active material powder such as cadmium oxide, for example,
is about 1 micron. A small amount of the commercially available
PTFE resin is dry-mixed macroscopically substantially uniformly
with a substantially greater amount of the cadmium oxide powder
using a mixing machine such as a conical blender or a V-blender. A
shear force is then applied to this powder mixture. This is
accomplished by the use of a pestle on the laboratory scale or a
pulverizing mill on the industrial scale. In this process step by
the application of sufficient shear force, the PTFE resin is
gradually formed into a fibrous network structure, which collects
the active material, promotes submicroscopic mixing, and forms
flock-like agglomerates. Since the formation of the agglomerates is
effected in the dry state, the energy for applying the shear force
acts in an efficient manner and the network structure is formed
with a reliably controlled strength. In this instance, the amount
of the PTFE resin powder is suitably 0.5-2%, preferably 1%, on the
basis of the amount of the active material. If the amount of the
resin powder is not greater than 0.5%, sufficient strength can not
be obtained. If the amount exceeds 2%, on the other hand, the range
of the PTFE resin inside the electrode increases and the reactivity
of the active material is lowered. Hence, the use of the excessive
resin powder should be avoided.
Next, a predetermined amount of an aqueous paste solution is added
to the abovementioned flock-like agglomerates and kneaded using a
kneading machine, such as a kneader. During this kneading step,
numerous and large and small flock-like agglomerates condense into
one rubbery mass. Namely, the added aqueous paste solution enters
the voids inside the agglomerates and functions to condense these
agglomerates into one rubbery mass.
The abovementioned aqueous paste solution is obtained by dissolving
a polymer paste in water. The viscosity of the resulting solution
is sufficiently within the range of from 50 to 3,000 cps. Though a
similar effect could be obtained by the use of an optional paste,
preferred are polyvinyl alcohol, polyethylene oxide, hydroxypropyl
cellulose and the like in view of the oxidation resistance of these
materials and their viscosity inside the battery. The added amount
of the paste may be only a trace (0.2-0.4 parts by weight per 100
parts by weight of the active material). In comparison with the
conventional production method of the paste type electrode wherein
the paste is used as a binder (2 part by weight of the paste per
100 parts by weight of the active material), therefore, the use of
the paste in such an amount hardly affects the battery performance.
The amount of water used as a solvent of the paste is preferably
such that it accounts for 60% of the void capacity of the
agglomerates. If it exceeds 80%, the water functions to unbind the
network structure of the PTFE resin fiber. On the contrary, if the
amount is less than 45%, it is difficult to form the rubbery
mass.
It is another important feature of the present invention that when
cadmium oxide is used as the active material, a condensation
oxyacid salt is dissolved in the abovementioned aqueous paste
solution.
As already described, during the production of a cadmium oxide
electrode using cadmium oxide as the starting material, cadmium
oxide readily reacts with water and is converted to cadmium
hydroxide having a lower density. Consequently, it is difficult to
obtain an electrode having a high energy density. In the present
invention it has been found, however, that conversion of cadmium
oxide to cadmium hydroxide can be prevented by adding a
condensation oxyacid salt to the water and dissolving it therein.
It is assumed that this effect arises from the function of the
condensation oxyacid salt as a negative catalyst during the
reaction between the water and cadmium oxide. Preferred examples of
the condensation oxyacid salt include disodium hydrogenphosphate
dodeca hydrate (Na.sub.2 HPO.sub.4.12H.sub.2 O), sodium
pyrophosphate, sodium hexametaphosphate, sodium orthosilicate,
disodium hydrogenarsenate and the like. The amount of added salt,
for example, in the case of disodium hydrogenphosphate dodeca
hydrate, is from 0.2 to 2.0% on the basis of the cadmium oxide
active material to obtain sufficient effect. It is confirmed that
such an amount of the salt hardly exerts any adverse influence on
the battery performance after the electrode is assembled in the
battery.
Since the liquid filling up the internal voids of the rubbery mass
has a remarkably high viscosity in comparison with water or an
organic solvent as mentioned already, the rubbery mass obtained in
this manner can easily be shaped into a sheet by means such as an
ordinary calender method. In other words, the liquid per se yet
remains as a non-compressive fluid and causes deformation depending
upon pressurizing. Nonetheless, since the liquor still maintains
its viscosity, it does not easily escape from the network structure
of the resin, thereby enabling to attain a uniform packed density.
Unlike the aforementioned U.S. Pat. Nos. 3,630,781 and 3,898,099,
the process of the invention adds the liquid in a predetermined
amount so that control of the density, etc. becomes easy and
workability becomes higher.
In the methods of the abovementioned prior art, all of mixing,
removal of the solution and application of the shear force are
carried out in the wet state. Consequently, loss of the raw
materials due to their sticking to the container, etc. is
inevitable in each of these steps. By contrast, in accordance with
the process of the present invention, the PTFE resin and the active
material are dry-mixed and the formation of the rubbery mass is
attained by perfect integration of the raw materials without
passing through the slurry state. Hence, there is no loss at all of
the raw materials.
Additionally, the surface of the electrode after completion of
drying in the present process is smooth without any tackiness due
to the addition of the paste and hence, the electrode is easy to
handle and provides another advantage with respect to the working
efficiency.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will be explained in further detail with
reference to an embodiment thereof.
EXAMPLE
To 100 parts by weight of an active material consisting of a 9:1
mixture (weight ratio) of a cadmium oxide powder and a metallic
cadmium powder is added 1 part by weight of a commercially
available unsintered PTFE powder and the materials are mixed by a
rotary dry mixer. Using a pulverizing mill, the mixture is
subjected to a shear force so as to cause the PTFE resin to form a
fibrous network and to form agglomerates of the active material
powder and the PTFE resin powder. An aqueous paste solution,
prepared separately by dissolving 0.2 parts by weight of
polyethylene oxide and 1 part by weight of disodium
hydrogenphosphate dodeca hydrate in 33.8 parts by weight of water
and having a viscosity of about 400 cps., is added to the
abovementioned agglomerates and kneaded inside a kneader to thereby
form a rubbery mass. The resulting rubbery mass is then shaped into
a 0.4 mm-thick sheet by an ordinary calender method and the sheet
thus obtained is attached onto both surfaces of an electrically
conductive core body consisting of a nickel-plated iron sheet and
dried at 80.degree. C. The thickness of the core body is then
reduced by a press roller so that the packed density becomes 2.7
g/cc calculated as metallic cadmium. The core body is subsequently
cut to a predetermined dimension to obtain a cadmium electrode.
Performance is compared between the cadmium electrode (A) obtained
in accordance with the process of the present invention, the
cadmium electrode (B) obtained by the method of U.S. Pat. No.
3,630,781 and the cadmium electrode (C) obtained in a customary
manner using a polymer paste such as methyl cellulose as a binder.
The results are illustrated in the following table.
______________________________________ Electrode A B C
______________________________________ Efficiency of active
material (0.2c standard), % 74.8 73.0 65.5 Energy density ("),
mAH/cc 908 750 685 *Equilibrium pressure (0.1c charging),
kg/cm.sup.2 0.7 1.3 4.8 *Ratio of capacity after 100 cycles to the
initial 103 100 89 capacity High rate discharging per- formance (4c
capacity/0.1c 74.0 70.5 59.2 capacity)
______________________________________
The test items marked with asterisks in the above table indicate
the performance of an enclosed type alkaline battery using the
electrode in combination with the ordinary sintered type nickel
hydroxide electrode.
It can be appreciated from the table above that in comparison with
the cadmium electrode (C) obtained by the ordinary production
method, the electrodes (A) and (B) respectively obtained by the
process of the present invention and the method of U.S. Pat. No.
3,630,781 exhibit drastic improvement in their performance.
In comparison with the cadmium electrode (B) of the U.S. Pat. No.
3,630,781, the cadmium electrode obtained by the present process
exhibits further dramatic improvement.
This improvement may be attributed to the following point.
According to the method of U.S. Pat. No. 3,630,781 as mentioned
already, the active material powder is mixed in the aqueous
dispersion of the fluorocarbon resin and the dispersion is then
subjected to breaking so that the active material and the
fluorocarbon resin are excessively homogenized. Due to the water
repellency inherent in the fluorocarbon resin, therefore, there
occurs a phenomenon similar to the film-forming phenomenon
occurring when a polymer paste is used as a binder, whereby the
contact of the active material with the electrolyte as well as with
the gas becomes inferior to the process of the present invention,
and the efficiency of the active material and the equilibrium
pressure also become inferior. Meanwhile, in the process of the
present invention, since the active material and the fluorocarbon
resin are mixed in the powder form and subjected to shear force,
there is not obtained an excessively homogeneous mixing. It is
assumed that the difference in the mixing method gives the
electrode of the present invention its better performance
characteristics.
Whereas the cadmium oxide active material reacts with water and is
easily converted to cadmium hydroxide of lesser packed density
during the mixing step with the aqueous dispersion and the step of
reducing the water content in the production method of U.S. Pat.
No. 3,630,781, the condensation oxyacid salt is added in the
process of the present invention to the aqueous paste solution to
be added to the agglomerates for forming the rubbery mass so that
it is possible to prevent conversion of cadmium oxide to cadmium
hydroxide and to accomplish remarkable improvement in the energy
density.
Due to the addition of the paste, the surface of the finished
electrode is smooth without tackiness, thereby improving the
operational efficiency of the subsequent assembly of the
battery.
As described in the foregoing paragraph, the present invention
provides a production process of an electrode for an alkaline
battery, said electrode having high energy density and good
mechanical strength, ensuring extremely high reactivity to the
active material and improving the workability. Hence, the process
of the present invention has extremely great industrial
advantage.
* * * * *